Method of controlling chromate conversion coating baths



Dec. 22, 1964 I w. c. JONES 3,162,207

MELIHOD 0F CONTROLLING CHROMATE CONVERSION COATING BATHS Filed April 5,1962 Q I ma I 9 i I a [m lr\ g Q 4k: $9M I w U R: IIIII I I N M IIIII II I w IIIIIIHI I:

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United States 3 162,207. METHOD OF CONTROLLING CHROMATE CON- VERSIONCOATING BATHS William C. Jones, Thompsonville, Conn., assignor to T heChemical Corporation, Springfield, Mass., a corporation of MassachusettsFiled Apr. 3, 1962, Ser. No. 184,779

1 Claim. (Cl. 137-5) This invention relates to chromate conversioncoating from liquid media and particularly to methods of automaticallycontrolling the effectiveness of the conversion coating bath. v

One of the principal problems in quality control of chromate conversioncoating processes is accurate quantitative analysis of the activematerial in the liquid media. For example, in the control of chromateconversion coating baths it is desirable to measure the amount of activechromium in the bath.

Chromate baths contain hexavalent chromium, from chromic acid, othermineral acids, activator and buffers. Conventionally, measurement of theamount of active material in a bath involves the ascertainment ofacidity by titration, the determination of the hexavalent chromiumcontent by titration, or the measurement of the pH of the bath or acombination of the above methods.

As a chromate conversion coating bath is used for coating variousarticles, the strength of the bath decreases through what is known asdrag in of Water and drag out, or removal of the coating material. Inaddition, the metal being coated dissolves to some extent in the bathgradually increasing in quantity as the bath is used. It has been foundthat the dissolved metal tends to associate itself with hexavalent oractive chromium in solution and in effect renders the chromium lessactive for coating purposes. It has also been learned that thehexavalent chromium degenerates to a reduced condition known astrivalent chromium which also tends to associate and interfere with thecoating effectiveness of hexavalent chromium.

As indicated above the conventional methods of measuring coatingeffectiveness are based on periodical batch type tests, one of which isa total acidity determination. It has been found that this test measuresnot only the presence of active acidic components which are effectivefor coating reaction but is alsoresponsive to the other dissolved metaland trivalent chromium in the bath. Thus, it will be realized that asthe solution becomes less effective the titration analysis of totalacidity becomes less accurate.

The accuracy of tests which involve determination of the amount ofhexavalent chromium in the bath is also adversely affected by thepresence of other dissolved metals and trivalent chromium in thesolution. The reason for this is that although the hexavalent chromiumis present in the bath its effectiveness is diminished proportionatelyto the amount of other dissolved metals and trivalent chromium present.

The other generally used method of controlling coating baths comprisesthe measurement of pH by electrometric methods. This is not asufficiently accurate method of control because pH, which is a logfunction, requires too large changes in total acidity to produce anysignificant measurable changes. This is particularly true becausechromate baths are operated in such low pH ranges, on the order of about1.5 or less.

It will be, of course, appreciated th'at the two titration proceduresdescribed above do not lend themselves to automatic control of coatingbath concentration, atid while pH controllers do not have this drawbacktheir lack of sensitivity in the operating ranges encountered in icechromium conversion coating also makes them unsuitable for accuratecoating control.

It is the principal object of this invention to provide an improvedmethod and means for automatically controlling the coating etfectivenessof chromate conversion coating baths.

The above and other objects and advantages of this invention will bemore readily apparent from the following description and with referenceto the accompanying drawing which is a circuit diagram showing one meansof carrying out this invention.

In accordance with this invention it has been found that electricconductivity, or its reciprocal, resistance, can be accurately andcontinuously determined and that this determination is an extremelyaccurate measure of the amount of active material in the chromate bath.It was found unexpectedly that chromate solution conductivity is afunction of the amount of available or efiective hexavalent chromiumpresent and not merely a measure of the amount of material dissolved inthe bath.

In order to evaluate the effectiveness of automatic solution control thefollowing experiment was conducted. A container of chromating solutionwas mixed having an operating level of chromium concentration suitablefor a yellow iridescent film. The conductivity of this solution wasmeasured and a zinc plated test panel was processed.

Thereafter a quantitycof zinc dust was added to the bath in smallincrements; the bath did not form a satisfactory coating on the plate.At this point the conductivity of the solution was again measured and adistinct decrease was noted. Thus it was learned that the conductivityvariation was responsive to diminution in coating effectiveness.

Following the low conductivity reading additional liquid concentrate ofchromating material was introduced into the bath sufiicient in quantityto restore the original conductivity. When the conductivity had beenrestored, the bath once more became eifectivefor depositing satisfactoryfilms on the metal plate. Zinc dust was again added and the process ofreducing and increasing coating effectiveness was repeated. It was notedthat successive reductions in effectiveness required less and less zincand correspondingly less and less concentrate was required to restorethe original conductivity. During the performance of these tests therewas no appreciable drag out of solution or evolution of gas, andactually the amount 'of dissolved metal in the bath increased ratherthan decreased due to the substantial quantities of zinc introduced intothe bath. Notwithstanding the overall increase in dissolved material,the conductivity decreased rather than increased when the bath becameless effective for coating purposes. This then constitutes an effectivemethod for indicating the coating effectiveness of a chromate bath.

in the figure is shown diagrammatically apparatus which may be used incarrying out this invention. This apparatus comprises a conductivitycell 2 which consists of two spaced metal plates or electrodes immersedin the liquid of the bath 6, contained in a tank 7. The electrodespreferably are disposed within an insulating chambcr 4 which isolates aportion of the liquid whereby resistance measurement is independent ofthe volume and location of the cells within the tank 7. The plates ofthe ceii are electrically connected by leads 9 to one arm of abridge-rectifier shown generally at 8. In this way variaticns in theconductivity of the bath may be measured as resistance variationsbetween the plates as compared with a resistance balance established bythe bridge.

An alternating current is connected to the bridge by means of leads 10and 12. The output from the bridge is connected by leads 14 and 16 to asuitable amplifying device which may be a magnetic amplifier 20. Themagnetic amplifier is a saturable reactor containing an iron core havinga primary winding which .receives the signal from the bridge circuit.The secondary coil receives the AC. voltage. The DC voltage input orcontrol signal increases the flux density of the coil with consequentamplification or increase in the output voltage. The AC. voltage issupplied to the amplifier by means of leads 22 and 24 connected to leadsand 12.

An output circuit from the magnetic amplifier is indicated generally at26 and asshown is in the form of a rectifier comprising diodes 28 and30. The cathodes of the diodes are connected together and'the junctionthereof is connected to one end of a load resistor 32. The other end ofthe load resistor is connected to a center tap of the secondary of themagnetic amplifier. The'voltage across the load resistor is rectifiedand applied across a filter capacitor 34 to a DC. sensing relay 36 whichoperates switch 38. Switch 38 in turn operates a relay 40 to energize afeed pump motor 42. A pump driven by the motor introduces a chromateconcentrate 43 into the tank 7.

In operation the cell 2 is immersed in the coating bath 6 mixed tocontain ingredients or amounts which will give optimum coating results.When the bath is in this condition a switch 44 is activated to energizethe apparatus including the bridge circuit 8. The outputi of the bridgeis balanced or nulled by adjustment of the rheostat arm 46, arms 48 and50 being the ratio'arms of the bridge. Each arm of the bridge contains adiode 52 arranged so that a rectified voltage output is obtained fromthe bridge.

When the bridge circuit is inv its null condition no output signal isreceived by the amplifier 20. However, when the effectiveness of thehexavalent chromium in the bath 6 is reduced, resistance between theplates of the cell 2 changes and the electrical balance of the bridge isdisrupted and a potential difference occurs at the output corners of thebridge. As a result a DC. current is introduced to the magneticamplifier by leads 14 and,16. This voltage generates an output signalfrom the amplifier 20 which is rectified by circuit 26. The rectifiedsignal operates relay switch 38 which in turn operates relay to startthe pump motor 42 and concentrate'is pumped into the tank 7 until thebridge circuit is restored to its null condition. I

' Operating lights 56 and 58 may be provided to indicate when thecircuit is energized and when the pump is operating.

Having thus described this invention what is claimed is:

In'the method of chromate conversion coating the steps which compriseforming a chromate conversion coating bath containing hexavalentchromium, the trivalent chrc' mium and the metal dissolved from thatbeing coated,"

electrically sensing the effective hexavalent chromium present in saidbath, obtaining an electrical signal which is a function of theconductivity of effective hexavalent chromium content of said bath,comparing said signal with a similar signal of predetermined vvalue, andintroducing into said bath in a response to deviation of said signalfrom said predetermined value a chromate concentrate'whereby said bathis continuously and automatically maintained at optimum coatingeffectiveness.

References Cited by the Examiner UNITED STATES PATENTS ISADOR WEIL,Primary Examiner. CLARENCE R. GORDON, Examiner.

